An example of offline skilling would be EvE Online. Your character (pilot) does not gain skills by getting experience. Instead, your character gains skillpoints towards a skill you are training whether you are in or out of game. Also, I am fairly certain that skills continue to build even when the game servers are down.
Anyone have any broad ideas of how they go about this, or better yet how I could go about implementing something like this in a game I am developing. I'm not looking for a detailed explanation here (don't let me hold you back though), but I am looking for a signpost that points me in the correct direction.